To clarify, the ‘it’ refers to a ‘wall’ not the money for a wall. Merely having a wall is not sufficient - it is a tool which must be actively used to keep unauthorized people out. Allowing fake asylum claimants through a ‘big beautiful door’ or not supporting deportation of those who do breach it, or allowing visa overstays, or myriad other enforcement failings would render it useless.
I agree with this for the most part. A barrier is STILL better than no barrier and would help channel folks to those big beautiful doors where the majority of the problem can be addressed. Enforcement is an issue all it's own. Right now, crossing our southern border is too easy and almost impossible to defend. A wall, properly constructed, would help greatly to reduce the influx of illegals.